Analog front-end (AFE) circuits are widely used in various electronic signal processing applications to provide signal conditioning for sensors and other circuits. For example, An AFE is often used for conditioning an analog signal encoding digital data for receipt and conversion by an analog-to-digital converter (ADC).
A simple AFE circuit 100 with variable gain for converting an analog signal into a digital signal is shown in FIG. 1. The AFE circuit 100 comprises a variable gain amplifier (VGA) block 110, a signal conditioning block 120, and an ADC block 130. A received analog signal 140 is first adjusted in amplitude by the VGA block 110 based on a gain control signal 150 either to increase (amplify) or decrease (attenuate) the analog signal 140 amplitude depending on the received signal characteristics and the desired signal amplitude in the following analog blocks. The amplitude-adjusted analog signal 160 is then received by the signal conditioning block 120 which conditions the amplitude-adjusted analog signal 160, for example by filtering noise, mitigating inter-symbol interference, compensating direct current (DC) offset, and improving signal integrity of the received signal. The signal conditioning block 120 may include one or more blocks, each of which may have nonlinear effects on the signal 160 and may amplify or attenuate the signal 160. The conditioned analog signal 170 is received by the ADC 130 which converts the conditioned analog signal 170 into a digital signal 180. Usually the calculated signal level at the output of the ADC 130 is used to set the gain control signal 150 of the VGA 110 so that the output signal level meets certain criteria.
One challenge faced in the design and implementation of an AFE circuit is gain compression which is illustrated in FIG. 2. As shown in FIG. 2, an ideal amplifier will be characterized by a linear relationship between an input voltage and an output voltage over all values of the input voltage. The ideal amplifier is thus said to be characterized by a linear gain. A practical amplifier, however, is typically characterized by nonlinear gain above some threshold amplitude which depends upon numerous factors including the characteristics of the amplifier circuitry and the environment of the circuit. Above the threshold input voltage, which may be termed a ‘nonlinear range’, further increases in input voltage do not result in proportional increases in output voltage, but rather typically result in smaller increases than produced below the threshold voltage in a ‘linear range’. Thus, above the threshold voltage, the gain of the amplifier is said to be ‘compressed’.
Gain compression is a challenge in the design and implementation of AFE circuits as the analog elements of an AFE circuit are typically characterized by nonlinear gain above a threshold signal amplitude (and thus, voltage). A received analog signal will typically include portions with relatively higher and lower instantaneous amplitude, for example the peaks and valleys, respectively, of a carrier wave. Depending upon the gain of the VGA block, the peaks of the signal may be made to fall within the nonlinear range of the signal conditioning block, while the valleys remain in the linear range. Due to gain compression, the peaks, and any other portion of the signal above the threshold amplitude, do not increase in amplitude as much as the valleys of the signal and any other portion below the threshold amplitude, thus causing distortion of the signal received by the ADC block. In order words, the impact of gain compression is that the signal received by the ADC block is distorted by the nonlinear gain of analog elements of the AFE circuit, and the signal level received by the ADC block is not as expected. These effects cause an increased amount of error after converting the analog signal into a digital representation which in turn results in a degradation of overall system performance.
There are several challenges compensating for analog nonlinearity and AFE gain control. One challenge is that the observed signal level at the output of the ADC does not indicate whether nonlinear effects are being produced in the system. Consequently, increasing the gain in order to reduce the ADC sampling error may actually increase the sampling error due to the degradation of the signal from nonlinear effects. Another challenge is that the actual gain produced by the VGA typically varies from device to device due to silicon process variations and also varying device operating conditions such as voltage and temperature, which makes direct calculation of the actual gain of the VGA impractical, which in turn makes impractical direct calculation of the gain compression in the circuit using only the observed output signal level of the ADC.
One typical approach to gain control of a nonlinear amplifier is to measure the output voltage and increase the gain until a target output voltage level is reached. A disadvantage of this approach is that it does not take gain compression of the amplifier into account and as such severe signal degradation can occur.
A variant of the above typical approach is to select the target output voltage level based on a determination of a typical linear range of the AFE circuit. Such approach does not take into account, however, the fact that that the actual gain characteristics and nonlinear range of AFE circuits change over silicon process variations and operating conditions such as temperature and voltage. Using a predetermined target amplitude can nevertheless result in severe signal distortion due to varying process and operating conditions.
Another issue is that a typical signal conditioning circuit may include multiple stages of elements which include additional gain controls and are characterized by nonlinear gain when the input signal from a preceding stage is too high. It is problematic and impractical to allocate gains to these stages without visibility of the compression of the signal causing a stage to be overdriven by the preceding stage resulting in signal distortion.
In view of the above disadvantages, typical AFE circuits are designed to minimize gain compression by limiting the VGA gain so as to ensure that the amplitude of the amplitude-adjusted analog signal falls within the linear region of the conditioning circuit block. Lacking an accurate knowledge of the gain characteristics of the conditioning circuit block, and in order to compensate for variations in the gain due to process variations and operating conditions however, and in order to ensure that the complete dynamic range of the signal can be compensated, the resulting conditioned analog signal is typically below a level which provides an optimum signal-to-noise ratio (SNR) at the ADC block. The result is a degradation of the system performance due to non-optimal receiver gain.
In other words, in a conventional AFE design, the linear gain range of an amplifier is often over-designed in order to minimize the compression effects in the receiver with the disadvantageous result of increased power consumption and amplifier design complexity.
There is thus material value in techniques which address, compensate, or provide a solution to the problem of, gain compression in AFE circuits.